1. Field of the Invention
The present invention relates to a control system of an automatic transmission.
2. Description of Related Art
Conventionally, as a learning control of a line pressure control of the hydraulic control system of an automatic transmission during a shift operation, it has been known that a time period of the shift operation is measured to be compared with a target time period for the shift operation. If the actual time period is longer than the target value, the line pressure is increased and, conversely, if the actual time period is shorter than the target value, the line pressure is decreased. (see Japanese Patent Publication No. 63-3183 issued in 1988).
Meanwhile, the learning control of the line pressure is carried out only in an up-shift operation, where the engine rotation speed is reduced, and not carried out in a down-shift operation. This is because there is no control of a draining speed when a shift valve is switched to drain a hydraulic fluid through an orifice in the down-shift operation. With such an automatic transmission, when a 3-4 clutch pressure is abruptly released in a 3-2 shift operation in which a 3-4 clutch as a first frictional element is released and a 2-4 brake as a second frictional element is engaged, a shift shock (sudden dragging) is produced. In order to eliminate such shift shocks, it is necessary to control a releasing pressure of the frictional element properly by extending the time period for shift operation. In other words, the releasing pressure should be controlled in accordance with a turbine torque.
It should be noted that a frictional coefficient of a material forming the frictional element is largely changed with age and varies depending on a product. Therefore, in order to control the shift shock successfully, the above factors should be taken into account.